Robert Janes

A combined ESR/X-ray diffraction study of the Y2O3–BaO–CuO phase diagram

Measurements at both X- and Q-band frequencies of various phases in the Y–Ba–Cu–O system are reported. Signals close to the free-spin region are attributed to localised Cu2+ species with g-value anisotropies noticeably dependent upon the local crystal structure. These materials include BaCuO2, Y2BaCuO5, Y2Cu2O5; characteristic g values are discussed in terms of crystal-field models for electronic structure.Importantly, we find that high purity YBa2Cu3O7–x(x≲ 0.1) superconducting samples (Tc= 92 K), prepared at lower temperature by solution or sol–gel techniques, have no intrinsic ESR signals in the free-spin region. We propose that this ‘ESR silence’ is, in fact, not due to the absence of intrinsic paramagnetism in these superconducting oxides, but rather due to the extremely efficient spin–lattice relaxation of the d-band conduction electrons. Here, electron spin relaxation in these metallic systems for T > Tc proceeds via an Elliott-type mechanism involving the modulation of spin–orbit interactions via itinerant conduction electrons. This efficient relaxation mechanism would lead to extremely large ESR linewidths, typically in the range 50–100 kG.

ESR studies of high critical-temperature superconductors :absorption at low magnetic fields

One of the most intriguing aspects of ESR studies of oxide superconductors is the presence of a broad, and intense, low-field microwave absorption which grows in intensity upon cooling the sample through the superconducting transition temperature (Tc). In the present investigations we exploit the low-field response as a screening technique for superconductivity in a range of oxide materials. The materials investigated, derivatives of the 90 K superconductor YBa2Cu3O7, were specifically chosen so that Tc could be drastically modified either by oxygen loss or cation substitution. In all of these materials a clear correlation exists between the onset temperature of the low-field absorption and the superconducting transition temperature, as gauged by both a.c. inductive and electrical resistivity measurements. The temperature range over which the superconductivity onset was detected by these complementary methods extended from ca. 90 K down to ca. 30 K in the case of a Zn-substituted YBa2Cu3O7. Measurements on the corresponding Bi–Sr–Ca–Cu–O system, which becomes superconducting for temperatures below 110 K, also illustrate that this absorption phenomenon is not unique to YBa2Cu3O7 and hence will have widespread applicability in detecting superconductivity.

Speciation modelling of the electroprecipitation of rare-earth cuprate and nickelate materials Speciation of aqueous solutions not at equilibrium

Films of insoluble metal hydroxide have been obtained by cathodic electroprecipitation from aqueous solutions containing the nitrate salts of a lanthanide (La3+ or Nd3+) and a transition-metal cation (Ni2+ or Cu2+). Manipulation of deposition parameters, such as deposition voltage, Va and solution composition, allows for extensive variation in mole fractions, x, of each metal in the final bimetallic hydroxide product. The variations in x are attributed to changes in the chemical environment within the Nernst layer at the electrode/solution interface, occurring in tandem with differences in the solution- and solid-phase speciation characteristics of the two metals and their hydroxides.The experimentally obtained compositions of the deposited solid oxyhydroxide have been correlated with thermodynamically derived overall formation constants, β, and calculations of concentration obtained using a full speciation analysis. This model can be used to correctly predict compositions, which is taken to verify the model of electroprecipitation used.

Reactions of AgI ions in alcohols after radiolysis at 77 K

Solutions of silver perchlorate in methanol, ethanol, ethane-1,2-diol, isopropyl alcohol, allyl and propargyl alcohols have been exposed to 60Co γ-rays at 77 K and the resulting radicals studied using ESR spectroscopy. In addition to ĊH2OH solvent radicals, methanolic solutions gave AgII centres and a number of discrete Ag° species. The latter exhibited a range of hyperfine couplings, reflecting various degrees of solvation. Their relative yields were shown to vary according to initial AgI concentration and with added sodium perchlorate. After annealing to ca. 120 K, Ag° gave way to the molecular cluster cations Ag+2, Ag2+3, Ag3+4 and Ag4+5, previously identified by ESR and optical spectroscopy in a range of systems and shown to be formed by the sequential addition of AgI ions. Ag4+5 formation was favoured either by high initial AgI or high counterion (nitrate or perchlorate) concentration. During the silver aggregation process the solvent radicals (CH2OH) decayed and the adduct [Ag . CH2OH]+ was formed [Aiso(Ag)= 128 G]. On annealing to the softening point of the matrix these species were lost. No higher-nuclearity molecular centres were detected. However, a weak singlet in the free-spin region grew in. This is tentatively assigned to conduction electron spin resonance (CESR) in pseudo-metallic, Ag0n particles. On further melting the Ag0n signal disappeared and the sample became black, indicating the eventual formation of colloidal silver. Similar reactions were observed in ethanol, ethane-1,2-diol and isopropyl alcohol.The allyl alcohol system did not give any clearly defined Ag° centres, only solvent radicals being observed at 77K. Two possible silver–solvent adducts were detected after annealing, one with a larger Aiso(Ag) value than expected (ca. 360 G). Results for propargyl alcohol were similar, except that only one silver–solvent species was formed. Possible structures for these centres are discussed. When both systems were annealed to the matrix softening point, the adduct signals decayed and an isotropic singlet in the free-spin region was revealed. This signal was particularly strong in the propargyl system. The g value and linewidth are similar to values recently reported for CESR signals from Ag0n latent-image particles formed in silver halide matrices by photolysis.

Speciation analysis applied to the electrodeposition of precursors of neodymium cuprate and related phases First application of speciation modelling to a solution not at equilibrium

We report the first time that speciation analyses have been used to explain the distribution of products obtained by a system not at equilibrium: films of insoluble metal hydroxide have been obtained by cathodic electroprecipitation from aqueous solutions containing the nitrate salts of Nd3+ and Cu2+. Variation in film composition is achieved by manipulating deposition parameters, such as deposition voltage Va and solution composition. The variations are caused by changes at the electrode/solution interface and differences in the solution- and solid-phase speciation characteristics of the two metals and their hydroxides.The experimental compositions of the Nd–Cu hydroxides are correlated with thermodynamic formation constants β, and with calculations of concentrations obtained using speciation analyses. The accuracy of the predicted compositions is taken to verify the model of electroprecipitation.

Radicals with one-electron silver-carbon bonds formed by ionizing radiation: An ESR study

Abstract The radicals Ag·CH 2 OH + and Ag·CHMeOH + have been prepared by low-temperature radiolysis of solutions of silver perchlorate in methanol and ethanol respectively, and characterised by ESR spectroscopy. These centres are thought to be produced by the reaction of matrix-derived radicals with silver ions. Interpretation of their ESR spectra was facilitated by using 13 C- and 2 H-labelled alcohols. The magnitudes of the hyperfine coupling constants give estimated spin densities of ⩾ 0.17 on silver and ≈ 0.5 on carbon.

Magnetic field dependent microwave absorption in high-Tc superconducting cuprates

A strong microwave absorption at low magnetic fields is observed for a variety of metallic cuprates below their critical superconducting transition temperatures (Tc) using conventional electron paramagnetic resonance (EPR) instrumentation. The low-field microwave signal was investigated in the following high-temperature superconductors, YBa2Cu3O7–x(Tc= 95 K), GdBa2Cu3O7–x(Tc= 95 K), YBa2Cu4O8(Tc= 75 K), (Bi1.6Pb0.4)Sr2Ca2Cu3Oy(Tc= 115 K), Pb0.5Bi2Sr2Ca2.5Cu4.5Oy(Tc= 118 K)(Tl0.5Pb0.5)Sr2(Ca0.8Er0.2)Cu2Oy(Tc= 115 K), Tl2Ba2Ca2Cu3Oy(Tc= 118 K), TbTl2Ba2Cu3Oy(Tc= 84 K), Tl2Ba2Ca2Cu3O10(single-crystal, Tc= 118 K) and Bi2Sr2CaCu2O8(single-crystal, Tc= 80 K). We have investigated the usefulness of the technique as a practical non-intrusive method for screening potentially superconducting samples, with particular emphasis on sensitivity, the effect of microwave power and the inherent problems of studying signals at small magnetic fields. In the latter regard, we have also carried out studies using a radiofrequency EPR spectrometer operating at 300 MHz, which facilitated detailed calibration of the 0–200 G region, where the signal is most intense.

Radical cation of HP(O)(OMe)2: electron paramagnetic resonance evidence for competition between bonding to solvent, a 1,2-hydrogen shift, and a 1,4-hydrogen shift

Exposure of dilute solutions of dimethylphosphonate in fluorotrichloromethane to 60Co γ-rays at 77 K gave EPR features characteristic of three types of radical cations. One is the parent cation which has formed a weak σ* bond between oxygen and chlorine, with Amax(35Cl)= 78 G. The second is the rearranged distonic cation H2ĊOPH(OMe)(OH)+ and the third is identified as the phosphoryl radical, (MeO)2P(OH)+.In more concentrated solutions (MeO)2PO radicals were also observed, and exposure to visible light converted the chlorine adducts into the carbon-centred radicals.

Low-field microwave absorption in mechanical mixes of superconducting YBa2Cu3O7 –x with Al2O3 and BaPbO3

Magnetic-field-dependent (X-band) microwave absorption in superconducting YBa2Cu3O7 –x, diluted into insulating Al2O3 and metallic BaPbO3 hosts, is reported here. X-Ray diffraction and a.c. susceptibility (mutual inductance) measurements were also carried out on the same samples to compare the sensitivity limits for detecting the presence of superconducting material. The intensity of the low-field microwave absorption and the mutual inductance signal increased linearly with increasing fraction of superconductor.

Electrochemical deposition of the hydroxide precursors to YBa2Cu3O7–δ and related phases

Thin films of hydroxide precursors of YBa2Cu3O7delta; (YBCO) have been electrodeposited from aqueous solution. The compositions of the films were tailored in two ways: firstly by adjusting the deposition voltage, and secondly by varying the relative amounts of precursor solutes in solution. The desired hydroxide precursor to YBa2Cu3O7delta; having the correct ratios of cation can be formed by electrodeposition at a potentiostatic voltage of 1.4V (vs. SCE) at a silver electrode immersed in an aqueous solution of composition Y(NO3)3=0.0125moldm3; Ba(NO3)2=0.150moldm3 and Cu(NO3)2=0.025moldm3.